hey i just bought a peavey valveking 212 yesterday from guitar center. it played fine in the store, but when i got it home and turned up the volume after a while it starting hissing or giving feedback when i was not playing. any ideas what it might be? its my first tube amp so i dont know much about the details of tube amps yet...
lol dude that's just because you were using a good amount of distortion... there's nothing wrong with a bit of hiss. Feedback is normal when you're not playing... and the feedback is because you prob. have it up louder now and because the walls are prob. closer in your house then the store.
Where you place it in your room/house could also be a factor. It could be getting interference from other electrical appliances in the house or even from the outlet you have it plugged into.

Try moving it to different areas to see if that changes anything.
~We Rock Out With Our Cocks Out!: UG Naked Club.~
Once in a blue moon, God reaches down from his lofty perch, points at an infant boy and proclaims, "This one shall have balls carved out of fucking granite."
I have the valveking 212 and i had just got it back from service(3rd time now) for a blown switch and some touch up on the solder was needed. I had the same problem when i got it home even though i let the tubes warm up the amp started to sound like i had unplugged my guitar and put my thumb on the jack, creating that loud distorted humming. Turns out the plug i was using may not have been grounded correctly, and yes turning it off and back on temporarily fixed it for a bit. I moved the amp to plugs that i knew were grounded for sure(older house, basement is redone with new electrical), and i no longer have that problem, knock on wood. I would suggest trying what i just said considering it is a valveking, followed by cables, then jacks, etc.